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Help - GPS offset at a location/drift when it didn't seem to be

niclariv

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Hi all

I'm pretty new to the Mavic, and finally got it back up in the air today after waiting for my Refresh (indoor crash.. long story I'll write about another time).

I've had something strange happen with the new one. I took it out to my parent's house for a flyover. I'd flown the old one there before and it was completely stable. This time when I fired it up I noticed that the home point was recorded as being a little to the side - on the roof instead of in the courtyard of their place (it's shaped like a capitol H without the top part). So I turned it off and on again (I haven't figured out if there's an alternative solution to correct it if the wrong home point is recorded, though in this case as you'll see below maybe that wouldn't have helped), and this time the home point appeared to be correct. When I took off the drone drifted - slowly but noticeably. I didn't risk flying it anywhere, just brought it back down and struggled to land it on the table I launched it from due to the continual drift.

Looking at the replay of the flight in the DJI Go 4 app, I can see all the drifting and I saw that the GPS never had full bars, so I thought maybe it was a bad GPS signal (though I wondered why it would have been perfect the other time I've flown in the exact same location).

But the weird thing is, when I looked back at the flight replay of the old flight at my parents', (the flight that was completely stable), it shows the entire flight a little to the left of where it actually took place, including the home point being on the roof, not in the middle of the little courtyard.

So maybe what's happening is that Apple Maps actually has the satellite image misaligned? And when I restarted the drone today and it recorded the home point that seemed to be correct, but there was instability when I took off, it actually had a bad GPS lock, and if I'd taken off right away the first time I started it up it would have flown perfectly even though it was showing the drone's location as a little to the left (exactly the same as the perfect flight weeks ago with the old drone).

Sorry if I'm not making sense here. What do you think? Misaligned maps? Or something else? I'll have to go back and try again, but I did calibrate the compass and then fire it up again to see where it records the home point, and once again it was on the roof.. Always in the same spot, except for the 2nd power up today - the one that I flew and it was unstable.

Here are screen shots of the flight log:

This is the one that drifted, but the home point seems to me to be in the right spot:
fe89f7c30de9819064592e1afb48247b.jpg


And this is the flight weeks ago that went perfectly, but looking at it the whole flight appears to be to the left of where it actually was (it was more centered over the house):
bb4509a8753749b7ecac3255d20ecba2.jpg


Sorry if I'm not writing this out clearly, but I'd really appreciate some feedback. I worry that if I take off with the home point appearing to be on the roof in the satellite image and it needs to RTH it will land on the roof. But if it's apple maps that is skewed, then I should be able to safely ignore the fact that the home point doesn't appear to be where the drone is actually taking off from.

Thanks in advance!
Nick


Sent from my iPad using MavicPilots
 
To add to this, the blue dot indicating my position according to my iPhone appears to agree more with Apple Maps (though of course the bigger blue circle is indicating that the location isn't accurate and I could be anywhere in there, but I was standing more or less where the blue dot is in those photos).


Sent from my iPad using MavicPilots
 
All those trees may be the problem. My guess is you had a poor GPS lock even though there were plenty of satellites. The drift is the big giveaway. It should be rock solid. It might also be a compass or IMU calibration issue. I personally wouldn't worry about perfectly aligning with the satellite photo. Those can be off a bit. Recalibrate and test again away from trees. I'll bet the drift goes away. Good luck!
 
Well it turns out that when my dad was checking his property lines with the city it seemed to him like the satellite image was a little off. So the evidence is definitely pointing towards the one flight I did where it the home point 'appeared' to be in the right place according to the satellite actually being a bad GPS lock, and the other times I turned it on and it found the home point as being on the roof it was actually right. I might have a chance to try again out there tomorrow, so if it shows the Mavic as being in the same place on the roof I'll lift off and see if it's stable. Thanks!
 
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